Merocyanine 540

Evaluation of the Physico-Chemical Properties of Liposomes Assembled from Bioconjugates of Anisic Acid with Phosphatidylcholine

The aim of this work was the evaluation of the physico-chemical properties of a new type of liposomes that are composed of DPPC and bioconjugates of anisic acid with phosphatidylcholine. In particular, the impact of modified anisic acid phospholipids on the thermotropic parameters of liposomes was determined, which is crucial for using them as potential carriers of active substances in cancer therapies. Their properties were determined using three biophysical methods, namely differential scanning calorimetry (DSC), steady-state fluorimetry and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). Moreover, temperature studies of liposomes composed of DPPC and bioconjugates of anisic acid with phosphatidylcholine provided information about the phase transition, fluidity regarding chain order, hydration and dynamics. The DSC results show that the main phase transition peak for conjugates of anisic acid with phosphatidylcholine molecules was broadened and shifted to a lower temperature in a concentration- and structure-dependent manner. The ATR-FTIR results and the results of measurements conducted using fluorescent probes located at different regions in the lipid bilayer are in line with DSC. The results show that the new bioconjugates with phosphatidylcholine have a significant impact on the physico-chemical properties of a membrane and cause a decrease in the temperature of the main phase transition. The consequence of this is greater fluidity of the lipid bilayer.

Inhibition of phagocytic activity of ARPE-19 cells by free radical mediated oxidative stress

Oxidative stress is a main factor responsible for key changes leading to the onset of age-related macular degeneration (ARMD) that occur in the retinal pigment epithelium (RPE), which is involved in phagocytosis of photoreceptor outer segments (POS). In this study, hydrogen peroxide (H2O2), H2O2 and iron ions (Fe) or rose Bengal (RB) in the presence of NADH and Fe were used to model free radical mediated oxidative stress to test if free radicals and singlet oxygen have different efficiency to inhibit phagocytosis of ARPE-19 cells. Free radical mediated oxidative stress was confirmed by HPLC-EC(Hg) measurements of cholesterol hydroperoxides in treated cells. Electron paramagnetic resonance (EPR) spin trapping was employed to detect superoxide anion. Cell survival was analyzed by the MTT assay. Specific phagocytosis of fluorescein-5-isothiocyanate-labeled POS and non-specific phagocytosis of fluorescent beads were measured by flow cytometry. HPLC analysis of cells photosensitized with RB in the presence of NADH and Fe indicated substantial increase in formation of free radical-dependent 7α/7β-hydroperoxides. EPR spin trapping confirmed the photogeneration of superoxide anion in samples enriched with RB, NADH and Fe. For all three protocols sub-lethal oxidative stress induced significant inhibition of the specific phagocytosis of POS. In contrast, non-specific phagocytosis was inhibited only by H2O2 or H2O2 and Fe treatment. Inhibition of phagocytosis was transient and recoverable by 24 h. These results suggest that free radicals may exert similar to singlet oxygen efficiency in inhibiting phagocytosis of RPE cells, and that the effect depends on the location where initial reactive species are formed.

Assembly of New Merocyanine Chromophores with a 1,8-Naphthalimide Core by a New Method for the Synthesis of the Methine Function

A new method for the synthesis of the monomethine group using nitro as a leaving group in an SN-Ar reaction is described. A series of novel merocyanine dyes has been synthesised and their photophysical properties have been elucidated. The longest wavelength absorption occurs in the range 519–619 nm and the molar absorptivities vary with the substituents and are in the range 1000–47700 L mol–1 cm–1. The dyes show high chemical and photostability. One example from the series has the ability to distinguish methanol from ethanol. The introduction of a quinoid fragment into the structure leads to a pronounced intramolecular charge transfer and hence a noticeable positive solvatochromism. The structures and electronic properties of the compounds have been studied by density functional theory (DFT) and time-dependent DFT.

Photosensitized oxidative stress to ARPE-19 cells decreases protein receptors that mediate photoreceptor outer segment phagocytosis

PURPOSE

To determine whether previously shown photodynamic (PD)-induced inhibition of specific photoreceptor outer segment (POS) phagocytosis by ARPE-19 cells is associated with reductions in receptor proteins mediating POS phagocytosis, and if PD treatment with merocyanine-540 (MC-540) produces additional effects leading to its inhibition of nonspecific phagocytosis.

METHODS

ARPE-19 cells preloaded with MC-540 or rose bengal (RB) were sublethally irradiated with green light. Phagocytosis of POS was measured by flow cytometry and POS receptor proteins (Mer tyrosine kinase receptor [MerTK] and integrin subunits αv and β5) and β-actin were quantified by Western blotting at 0.5 and 24 hours after irradiation, with comparison to samples from nonsensitized control cultures. The intact integrin heterodimer αvβ5 was quantified by immunoprecipitation followed by blotting. The distribution of N-cadherin, ZO-1, and F-actin was visualized by fluorescence microscopy.

RESULTS

Mild PD stress mediated by both photosensitizers that elicits no significant morphologic changes produces transient and recoverable reductions in MerTK. The individual αv and β5 integrin subunits are also reduced but only partially recover. However, there is sufficient recovery to support full recovery of the functional heterodimer. Light stress mediated by MC-540 also reduced levels of actin, which is known to participate in the internalization of particles regardless of type.

CONCLUSIONS

After PD treatment POS receptor protein abundance and phagocytosis show a coincident in time reduction then recovery suggesting that diminution in receptor proteins contributes to the phagocytic defect. The additional inhibition of nonspecific phagocytosis by MC-540-mediated stress may result from more widespread effects on cytosolic proteins. The data imply that phagocytosis receptors in RPE cells are sensitive to oxidative modification, raising the possibility that chronic oxidative stress in situ may reduce the efficiency of the RPE's role in photoreceptor turnover, thereby contributing to retinal degenerations.

Sub-lethal photodynamic damage to ARPE-19 cells transiently inhibits their phagocytic activity

Efficient phagocytosis of photoreceptor outer segments (POS) membranes by retinal pigment epithelium (RPE) plays a key role in biological renewal of these highly peroxidizable structures. Here, we tested whether photodynamic treatment, mediated by merocyanine 540 (MC 540), rose Bengal or a zinc-substituted chlorophyllide inhibited phagocytic activity of ARPE-19 cells in vitro. Specific phagocytosis of fluorescein-5-isothiocyanate-labeled POS isolated from cow retinas and nonspecific phagocytosis of fluorescent polystyrene beads were measured by flow cytometry. Photodynamic treatment, mediated by all three photosensitizers with sub-threshold doses, induced significant inhibition of the cell-specific phagocytosis. The nonspecific phagocytosis was inhibited by photodynamic treatment mediated only by MC 540. The inhibition of phagocytosis was a reversible phenomenon and after 24 h, the photodynamically treated cells exhibited phagocytic activity that was comparable with that of untreated cells. This study provides proof of principle that sub-threshold photodynamic treatment of ARPE-19 cells with appropriate photosensitizers is a convenient experimental approach for in vitro study of the effects of oxidative stress on specific phagocytic activity of RPE cells. We postulate that oxidative damage to key components of the cell phagocytic machinery may be responsible for severe impairment of its activity, which can lead to retinal degeneration.

Photodynamic action of merocyanine 540 on Staphylococcus epidermidis biofilms

Photodynamic treatment (PDT) has been proposed as a new approach for inactivation of biofilms associated with medical devices that are resistant to chemical additives or biocides. In this study, we evaluated the antimicrobial activity of merocyanine 540 (MC 540), a photosensitizing dye that is used for purging malignant cells from autologous bone marrow grafts, against Staphylococcus epidermidis biofilms. Effect of the combined photodynamic action of MC 540 and 532 nm laser was investigated on the viability and structure of biofilms of two Staphylococcus epidermidis strains, RP62A and 1457. Significant inactivation of cells was observed when biofilms were exposed to MC 540 and laser simultaneously. The effect was found to be light dose-dependent but S. epidermidis 1457 biofilm proved to be slightly more susceptible than S. epidermidis RP62A biofilm. Furthermore, significant killing of both types of cells was attained even when a fixed light dose was delivered to the biofilms. Confocal laser scanning microscope (CLSM) analysis indicated damage to bacterial cell membranes in photodynamically treated biofilms, while disruption of PDT-treated biofilm was confirmed by scanning electron microscopy (SEM).

Controlled cooling during semen cryopreservation does not induce capacitation of spermatozoa from two portions of the boar ejaculate

Cryopreservation imposes dramatic changes in boar sperm survivability but it is as yet unclear which part of the process affects the spermatozoa the most. The present study monitored, along the entire process of cryopreservation, the stability (PMS) of the architecture of the lipid plasma membrane and its integrity (PMI), as well as the kinetics of the processed spermatozoa using two portions from the boar ejaculate (P1 = the first 10 mL of the sperm-rich fraction, SRF; P2 = the rest of the ejaculate), frozen in a recently developed package, the MiniFlatPack (MFPs, 0.5 x 10(9) sperm/dose). Evaluation was made at four specific stages, viz. S1 = after collection (suspended in Beltsville thawing solution, BTS); S2 = at 15 degrees C (suspended in lactose-egg yolk, LEY); S3 = at 5 degrees C (suspended in LEY plus glycerol); and S4 = post-thaw. Both sperm kinetics (using computer-assisted sperm analysis, CASA) and PMS [i.e. the degree of lipid disorder and of the exteriorization of phosphatidylserine (PS) in the plasma membrane, measured by flow cytometry using Merocyanine-540 (M-540), and Annexin-V (AV) respectively], as well as plasma membrane integrity [PMI, i.e. the degree of membrane damage, measured using Yo-Pro-1 or propidium iodide (PI)] were assessed after incubation in BTS at 38 degrees C. Moreover, spermatozoa were challenged by incubation in modified Brackett-Oliphant medium (mBO+) with 37 mm of bicarbonate at 38 degrees C for 30 min, and their PMS and PMI further explored. Total sperm motility was significantly higher in P1 than in P2 along the entire process (S1-S4; p < 0.01), decreasing significantly at S4 for both fractions (p < 0.0001). The proportion of spermatozoa showing linear motility (LinM) was similar between ejaculate portions (P1 and P2), with a significant increase post-thaw (S4; p < 0.0001). During cooling (S1-S3) but not post-thaw (S4), lateral head displacement (LHD) differed between portions and changed along the stages (p < 0.01). Sperm velocity differed between portions in S1 (p < 0.01), but remained similar, independently of the portion, thereafter (S2-S4). Both PMS and the total number of live spermatozoa remained similar between S1 and S3 while incubated in BTS for both ejaculate portions. Sperm mortality increased post-thaw (S4) in both portions but the degree of lipid disorder remained low in the live cells (1.28% for P1; 1.55% for P2). Exposure to mBO+, on the other hand, significantly increased membrane lipid disorder along cooling (S1-S3; p < 0.0001), increasing the percentages of dead spermatozoa, especially post-thaw (around 70%, both portions). PS-exteriorization (AV) was not evident along the cryopreservation process in control (BTS) samples and exposure to mBO+ only induced minor variations. The data showed that kinetics, PMS and PMI of boar spermatozoa suspended in BTS (S1), LEY (S2) or LEY plus glycerol (S3) were maintained during controlled cooling but were altered by thawing, showing more characteristics of cell injury than of sperm capacitation. The spermatozoa were able to capacitate but the bicarbonate challenge destabilized the plasma membrane during initial cooling and accelerated membrane changes post-thaw. We conclude that capacitation of boar spermatozoa does not occur during controlled cooling.

Experimental and DFT studies: novel structural modifications greatly enhance the solvent sensitivity of live cell imaging dyes

Structural modifications of previously reported merocyanine dyes (Toutchkine, A.; Kraynov, V.; Hahn, K. J. Am. Chem. Soc. 2003, 125, 4132-4145) were found to greatly enhance the solvent dependence of their absorbance and fluorescence emission maxima. Density functional theory (DFT) calculations have been performed to understand the differences in optical properties between the new and previously synthesized dyes. Absorption and emission energies were calculated for several new dyes using DFT vertical self-consistent reaction field (VSCRF) methods. Geometries of ground and excited states were optimized with a conductor-like screening model (COSMO) and self-consistent field (SCF) methods. The new dyes have enhanced zwitterionic character in the ground state and much lower polarity in the excited state, as shown by the DFT-VSCRF calculations. Consistently, the position of the absorption bands are strongly blue-shifted in more polar solvent (methanol compared to benzene), as predicted by the DFT spectral calculations. Inclusion of explicit H-bonding solvent molecules within the quantum model further enhances the predicted shifts and is consistent with the observed spectral broadening. Smaller but significant spectral shifts in polar versus nonpolar solvent are predicted and observed for emission bands. The new dyes show large fluorescence quantum yields in polar hydrogen-bonding solvents; qualitatively, the longest bonds along the conjugated chain at the excited S1 state minimum are shorter in the more polar solvent, inhibiting photoisomerization. The loss of photostability of the dyes is a consequence of the reaction with and electron transfer to singlet oxygen, starting oxidative dye cleavage. The calculated vertical ionization potentials of three dyes I-SO, AI-SO(4), and AI-BA(4) in benzene and methanol are consistent with their relative photobleaching rates; the charge distributions along the conjugated chains for the three dyes are similarly predictive of higher reaction rates for AI-SO(4) and AI-BA(4) than for I-SO. Time-dependent DFT calculations were also performed on AI-BA(4); these were less accurate than the VSCRF method in predicting the absorption energy shift from benzene to methanol.

A new hat for an old enzyme: Waste management

The history of research regarding secretory phospholipase A(2) (sPLA(2)) has often focused in one of two directions. Originally, the enzyme was studied biophysically in terms of its fundamental structure, enzymology, and the relationship between membrane physics and catalytic activity. More recently, a large and growing body of information has accumulated concerning regulatory factors, tissue distribution, and physiological/pathological roles of sPLA(2). Evidence is presented that suggests an additional function for the protein in which it helps to clear dead and damaged cells while avoiding digestion of those that are healthy. Apparently, the ability of the enzyme to discriminate between susceptible and resistant cells depends on physical properties of membrane lipids related to order, distribution, and neighbor/neighbor interactions. Investigations into this action of the enzyme offer the rare opportunity to apply biophysical approaches and principles to a physiological setting.

Capacitation-like changes in equine spermatozoa following cryopreservation

The primary objective of this study was to assess plasma membrane characteristics and activation of signal transduction pathways in equine spermatozoa during both in vitro capacitation and cryopreservation. Significant plasma membrane restructuring, as assessed by measurement of plasma membrane lipid disorder and phospholipid scrambling, was not observed until after cryopreservation and subsequent thawing (P < 0.05). Although in vitro capacitated cells also displayed increased plasma membrane lipid disorder and phospholipid scrambling (P < 0.05), it appeared that regulation of these events in in vitro capacitated versus cryopreserved equine spermatozoa was not identical. Addition of 5 microM staurosporine to the capacitation media reduced plasma membrane phospholipid scrambling (P < 0.05), but supplementation to the freezing extender prior to cryopreservation did not. Furthermore, progesterone was able to induce a greater degree of acrosomal exocytosis in in vitro capacitated versus frozen/thawed spermatozoa. Expression of phospholipid scramblase, a protein thought to be important in plasma membrane phospholipid scrambling, did not differ between treatments. Comparison of protein tyrosine phosphorylation patterns between in vitro capacitated and cryopreserved cells demonstrated a divergence in signal transduction. Cellular signaling in in vitro capacitated equine spermatozoa appeared to be in part dependent on activation of the cAMP/PKA pathway, whereas signaling in cryopreserved cells seemed to proceed predominantly through alternative pathways. Taken together, these data support the idea that capacitation and "cryocapacitation" are not equivalent processes.

Functional consequences of oxidative membrane damage

The interaction of reactive oxygen species with biological membranes is known to produce a great variety of different functional modifications. Part of these modifications may be classified as direct effects. They are due to direct interaction of the reactive species with the molecular machinery under study with a subsequent chemical and functional modification of these molecules. An important part of the observed functional modifications are, however, indirect effects. They are the consequence of an oxidative modification of the environment of biological macromolecules. Lipid peroxidation-via its generation of chemically reactive products-contributes to the loss of cellular functions through the inactivation of membrane enzymes and even of cytoplasmic (i.e., water soluble) proteins. Oxidation of membrane lipids may, however, also increase the efficiency of membrane functions. This was observed for a series of transport systems. Lipid peroxidation was accompanied by activation of certain types of ion channels and ion carriers. The effect is due to an increase of the polarity of the membrane interior by accumulation of polar oxidation products. The concomitant change of the dielectric constant, which may be detected via the increase of the membrane capacitance, facilitates the opening of membrane channels and lowers the inner membrane barrier for the movement of ions across the membrane. The predominant effect, however, at least at a greater extent of lipid peroxidation, is the inhibition of membrane functions. The strong increase of the leak conductance contributes to the depolarization of the membrane potential, it destroys the barrier properties of the membrane and it may finally lead, via an increase of cytoplasmic Ca(2+) concentration, to cell death. The conclusions were derived from experiments performed with different systems: model systems in planar lipid membranes, native ion channels either reconstituted in lipid membranes or investigated in their natural environment by the patch-clamp method, and two important ion pumps, the Na/K-ATPase and the sarcoplasmic reticulum (SR) Ca-ATPase.

Mechanisms governing the level of susceptibility of erythrocyte membranes to secretory phospholipase A2

Although cell membranes normally resist the hydrolytic action of secretory phospholipase A(2) (sPLA(2)), they become susceptible during apoptosis or after cellular trauma. Experimentally, susceptibility to the enzyme can be induced by loading cells with calcium. In human erythrocytes, the ability of the calcium ionophore to cause susceptibility depends on temperature, occurring best above approximately 35 degrees C. Considerable evidence from experiments with artificial bilayers suggests that hydrolysis of membrane lipids requires two steps. First, the enzyme adsorbs to the membrane surface, and second, a phospholipid diffuses from the membrane into the active site of the adsorbed enzyme. Analysis of kinetic experiments suggested that this mechanism can explain the action of sPLA(2) on erythrocyte membranes and that temperature and calcium loading promote the second step. This conclusion was further supported by binding experiments and assessment of membrane lipid packing. The adsorption of fluorescent-labeled sPLA(2) was insensitive to either temperature or ionophore treatment. In contrast, the fluorescence of merocyanine 540, a probe sensitive to lipid packing, was affected by both. Lipid packing decreased modestly as temperature was raised from 20 to 60 degrees C. Calcium loading enhanced packing at temperatures in the low end of this range, but greatly reduced packing at higher temperatures. This result was corroborated by measurements of the rate of extraction of a fluorescent phosphatidylcholine analog from erythrocyte membranes. Furthermore, drugs known to inhibit susceptibility in erythrocytes also prevented the increase in phospholipid extraction rate. These results argue that the two-step model applies to biological as well as artificial membranes and that a limiting step in the hydrolysis of erythrocyte membranes is the ability of phospholipids to migrate into the active site of adsorbed enzyme.

Merocyanine 540-sensitized photokilling of leukemia cells: role of post-irradiation chain peroxidation of plasma membrane lipids as revealed by nitric oxide protection

The lipophilic dye merocyanine 540 (MC540) localizes primarily in the plasma membrane (PM) of tumor cells, where it can sensitize lethal photoperoxidative damage of potential therapeutic importance. We postulated (i) that chain peroxidation triggered by iron-catalyzed turnover of nascent hydroperoxides (LOOHs) generated by singlet oxygen ((1)O(2)) attack on PM lipids contributes significantly to overall cytolethality, and (ii) that nitric oxide (NO), a known scavenger of organic free radicals, would suppress this and, thus, act cytoprotectively. In accordance, irradiation of MC540-sensitized L1210 cells produced 5alpha-OOH, a definitive (1)O(2) adduct of PM cholesterol, which decayed during subsequent dark incubation with appearance of other signature peroxides, viz. free-radical-derived 7alpha/beta-OOH. Whereas chemical donor (SPNO or SNAP)-derived NO had little or no effect on post-irradiation 5alpha-OOH disappearance, it dose-dependently inhibited 7alpha/beta-OOH accumulation, consistent with interception of chain-carrying radicals arising from one-electron reduction of primary LOOHs. Using [(14)C]cholesterol as an L1210 PM probe, we detected additional after-light products of chain peroxidation, including diols (7alpha-OH, 7beta-OH) and 5,6-epoxides, the yields of which were enhanced by iron supplementation, but strongly suppressed by NO. Correspondingly, photoinitiated cell killing was significantly inhibited by NO introduced either immediately before or after light exposure. These findings indicate that prooxidant LOOH turnover plays an important role in photokilling and that NO, by intercepting propagating radicals, can significantly enhance cellular resistance.

pH, serum proteins and ionic strength influence the uptake of merocyanine 540 by WiDr cells and its interaction with membrane structures

It has been suggested that selective uptake of photosensitizers is due to significantly lower pH of the interstitial fluid in tumors compared to normal tissue. Therefore, the cellular uptake of merocyanine 540 (MC 540) was examined at two pH values: 6.8+/-0.1 and 7.4+/-0.1. There was no difference in spectral properties (absorption and fluorescence maxima positions, fluorescence intensity) of the drug in the presence of increasing amounts of either human blood plasma or FCS (0-2%) at the two pH values investigated. Nevertheless, significantly higher amounts of the drug were taken up by WiDr cells at pH 6.8+/-0.1, both in the presence of 10% FCS and in the absence of FCS. The absorption spectra of MC 540 in the presence of egg phosphatidylcholine (PC) liposomes turned out to be NaCl concentration-dependent (0.00-0.30 mol l(-1)). Membrane fluidity, as measured by fluorescence anisotropy of diphenylhexatriene (DPH), was unchanged within the experimental error in the NaCl concentration range 0.01-0.30 mol l(-1). The spectral changes indicated an enhancement of the incorporation of MC 540 into lipid membranes with increasing ionic strength. Such a salt concentration dependence suggests a possible involvement of the surface potential in the interaction of MC 540 with lipid membranes. The results might provide an explanation of the pH dependency of the cellular uptake of MC 540 observed in this study.

Preferential inactivation of paediatric solid tumour cells by sequential exposure to Merocyanine 540-mediated photodynamic therapy and Edelfosine: implications for the ex vivo purging of autologous haematopoietic stem cell grafts

Paediatric solid tumours exhibit steep dose-response curves to alkylating agents and are therefore considered candidates for high-dose chemotherapy and autologous stem cell support. There is growing evidence that autologous stem cell grafts from patients with solid tumours are frequently contaminated with live tumour cells. The objective of this study was to perform, in a preclinical purging model, an initial assessment of the safety and efficacy of a two-step purging procedure that combined Merocyanine 540-mediated photodynamic therapy (MC540-PDT) with a brief exposure to the alkyl-lysophospholipid, Edelfosine. Human and murine bone marrow cells and Neuro-2a murine neuroblastoma, SK-N-SH human neuroblastoma, SK-ES-1 and U-2 OS human osteosarcoma, G-401 and SK-NEP-1 human Wilms' tumour, and A-204 human rhabdomyosarcoma cells were exposed to a fixed dose of MC540-PDT followed by a brief incubation with graded concentrations of Edelfosine. Survival was subsequently assessed by in vitro clonal assay or, in the case of CD34-positive haematopoietic stem cells, by an immunohistochemical method. Combination purging with MC540-PDT and Edelfosine depleted all tumour cells by >4 log while preserving at least 15% of murine granulocyte/macrophage progenitors (CFU-GM), 34% of human CFU-GM, and 31% of human CD34-positive cells. The data suggest that combination purging with MC540-PDT and Edelfosine may be useful for the ex vivo purging of autologous stem cell grafts from patients with paediatric solid tumours.

A spectroscopic study of the fluorescence quenching interactions between biomedically important salts and the fluorescent probe merocyanine 540

The effects of several biologically important inorganic salts, including NaCl, NaI, NaBr, KCl, MgCl2, MgSO4 and CaCl2 on the electronic absorption and fluorescence spectra of Merocyanine 540 (MC-540) have been investigated in aqueous media at 25 degrees C. Depending on both the MC-540 concentration and the nature of salt, a new absorption band appears at about 515 nm, above the critical salt concentration (CSC), corresponding to salt-induced MC-540 aggregation. Several types of MC-540 fluorescence quenching by the salts are observed, according to their cationic charge and the nature of anion: in the case of monovalent ions (Na+, K+), a non-linear Stern-Volmer behaviour is observed, indicating variable contributions of dynamic and static quenching mechanisms, whereas for divalent alkaline-earth (Mg2+, Ca2+) ions, linear Stern-Volmer relationships are obtained. Using these results, an analytical quenchofluorimetric approach is proposed for the determination of magnesium ions.

Reactive oxygen-dependent production of novel photochemotherapeutic agents

The reactive nature of species derived from oxygen, such as singlet oxygen and hydrogen peroxide, has been exploited in the clinical setting for targeting bacteria, viruses, and tumor cells by photodynamic excitation of a variety of chromophores. This modality, termed photodynamic therapy (PDT), is currently being used to treat some forms of cancer. However, the applicability of conventional PDT is limited due to the absolute dependence on simultaneous exposure of the target to the photoactive compound and light. In 1990, we demonstrated that the need for simultaneous exposure of the biological target to light and photosensitizer could be circumvented by prior exposure (activation) of the sensitizer molecule to light and its subsequent use as any other anti-cancer or anti-viral drug. By dint of the nature of the protocol, this process was termed preactivation. Since then, the generation of biologically active molecules in vitro by preactivation has been validated using a variety of chromophores, such as merocyanine 540, Photofrin II, and naphthalimide. Here we briefly review the role of reactive oxygen species in the photodynamic effect, and provide an explanation for the mechanism of preactivation. We propose that photo-oxidation not only provides a novel means for the generation of biologically active molecules, but could also explain, at least in part the mechanism of conventional PDT. It is likely that the light-dependent breakdown of the chromophore to generate novel active compounds, in addition to reactive oxygen species, also contributes to the photodynamic damage observed on simultaneous exposure of the chromophore and target tissue to light during PDT.-Pervaiz, S. Reactive oxygen-dependent production of novel photochemotherapeutic agents.

Spectral imaging of MC540 during murine and human colon carcinoma cell differentiation

We studied the staining pattern of merocyanine 540 (MC540) by spectral imaging of murine CT26 and human HT29 colon carcinoma cells incubated with the dye MC540. This dye, usually considered a potential membrane probe, localized mainly in the cytoplasmic vesicles of the colon carcinoma cells. However, in cells incubated in an environment similar to that of a tumor (pH 6.7), high fluorescence was detected in the nuclear membrane and nucleoli. Under these acidic conditions, resembling the Krebs effect, a population of CT26 cells displayed fluorescent plasma membranes. In differentiating cells, exhibiting cell cycle arrest at G(1)-phase and an elevated level of alkaline phosphatase, MC540 fluorescence was confined to cytoplasmic vesicles and was not detected in the plasma membrane or in the nucleoli. Cell sorting analysis of both cell types at pH 5.0 revealed higher fluorescence intensity in proliferating cells compared to differentiating cells. The fluorescence intensity of MC540-stained cells reached a maximum at pH 5.0, although the fluorescence of MC540 dye was maximal at pH 7.2. This phenomenon may result from increased binding of MC540 monomers to the cells because disaggregation of the dye with Triton X-100 produced similar results. We conclude that nucleolar localization of MC540 and an elevated fluorescence intensity can be used as indicators for proliferating cells in the characteristically acidic tumor environment. (J Histochem Cytochem 49:147-153, 2001)

Subcellular localization of merocyanine 540 (MC540) and induction of apoptosis in murine myeloid leukemia cells

Subcellular localization of photosensitizers is thought to play a critical role in determining the mode of cell death after photodynamic treatment (PDT) of leukemia cells. Using confocal laser scanning microscopy and fluorescent organelle probes, we examined the subcellular localization of merocyanine 540 (MC540) in the murine myeloid leukemia M1 and WEHI 3B (JCS) cells. Two patterns of localization were observed: in JCS cells, MC540 was found to localize on the plasma membrane and mitochondria; and in M1 leukemia cells, MC540 was found to localize on lysosomes. The relationship between subcellular localization of MC540 and PDT-induced apoptosis was investigated. Apoptotic cell death, as judged by the formation of apoptotic nuclei, was observed 4 h after irradiation in both leukemia cell lines. Typical ladders of apoptotic DNA fragments were also detected by DNA gel electrophoresis in PDT-treated JCS and M1 cells. At the irradiation dose of 46 kJ/m2 (LD90 for JCS and LD86 for M1 cells), the percentage of apoptotic JCS and M1 cells was 78 and 38%, respectively. This study provided substantial evidence that MC540 localized differentially in the mitochondria, and the subsequent photodamage of the organelle played an important role in PDT-mediated apoptosis in myeloid leukemia cells.

Photocytotoxic and DNA damaging effect of temoporfin (mTHPC) and merocyanine 540 (MC540) on nasopharyngeal carcinoma cell

Photodynamic therapy (PDT) is a new approach to cancer treatment for a variety of malignant tumors. In this study, two clinical photosensitizers, Temoporfin (meta-tetra-hydroxyl-phenyl-chlorin; mTHPC) and merocyanine 540 (MC540), were selected to explore for their photocytotoxic and genotoxic effects on nasopharyngeal carcinoma cells (NPC/HK1 and CNE2). Results of tetrazolium reduction assay showed that 80% cell killing were achieved for both cell lines at 0.4 microg/ml mTHPC for 24 h incubation and then with 40 kJ/m2 light irradiation, whereas 40 microg/ml MC540 with 50 kJ/m2 light dosage was required to attain the same level of phototoxicity for NPC/HK1. On the contrary, NPC/CNE2 was quite resistant to MC540. Hence, mTHPC-mediated PDT exerted a more potent effect than MC540-mediated PDT, even though the molar extinction coefficient of the main absorption peak for MC540 is much higher than that of mTHPC. Dark cytotoxicity remained negligible for both sensitizers. Comet assay was used to evaluate the DNA strand break and potential genotoxic effect induced by mTHPC and MC540 on the NPC cells. No DNA strand break was detected in the absence of light, and under sublethal treatment (LD25) for either sensitizer-loaded cells. Confocal laser scanning microscopy showed that mTHPC and MC540 localized in the cytoplasm but not in the nucleus of the tumor cells, which provided evidence for undetectable DNA damage under dark and low photodynamic dose.

Photoactivated virucidal properties of tridentate 2,2'-dihydroxyazobenzene and 2-salicylideneaminophenol platinum pyridine complexes

The potent photoactivated virucidal activity of tridentate 2,2'-dihydroxyazobenzene- and 2-salicylideneaminophenol platinum pyridine complexes 1, 2, 4, 6, 7, 9, and 10 against enveloped viruses (e.g., EIAV, HIV, and HSV) is described.

Hemin-enhanced resistance of human leukemia cells to oxidative killing: antisense determination of ferritin involvement

Human HL-60 cells exhibited a strong hyperresistance to the lethal effects of photodynamic activity (singlet oxygen) or glucose oxidase activity (hydrogen peroxide) 16-20 h after being exposed to hemin (ferriprotoporphyrin IX). Hyperresistance was accompanied by the overproduction of immunodetectable ferritin, predominantly the heavy (H) subunit, which exhibits ferroxidase activity. Cells that had been enriched in apoferritin via pinocytotic uptake showed similar hyperresistance to both types of oxidative challenge. On the other hand, preincubating cells with hemin in the presence of a phosphorothioate-linked antisense oligodeoxynucleotide against H-ferritin mRNA resulted in a strong diminution in both hyperresistance and H-ferritin induction. No effects were seen when a scrambled order oligodeoxynucleotide of the same base composition was used, confirming that the antisense oligomer had specifically inhibited H-ferritin translation. These results indicate that induced ferritin played a crucial role in the observed cytological responses. Enhanced oxidant resistance is attributed to the ability of this ferritin to rapidly sequester and incapacitate redox-active iron.

A study of the binding of merocyanine 540 to myeloid leukemia M1 cells using an intensified charge-coupled device for fluorescence imaging microscopy

The binding of merocyanine 540 (MC540) to murine myeloid leukemia (M1) cells and normal erythrocytes was measured by fluorescence digital imaging microscopy using an intensified charge-coupled device. It was found that, on average, about three times more MC540 were bound to a unit membrane area of M1 cells than erythrocytes, a result consistent with previous studies. However, it was shown for the first time that MC540 binding varied significantly from one M1 cell to the next, and about 15% of the sensitized M1 cells were as MC540-negative as normal erythrocytes. Using the leukemic inhibitory factor as a differentiation inducer, M1 cells were induced to differentiate into mature macrophage-like cells in vitro. Such treatment lowered the average MC540 binding by about one-third but did not affect the cell-to-cell variation significantly.

Alterations in plasma membrane of glioblastoma cells by photodynamic action of merocyanine 540

Photodynamic action of merocyanine 540 (MC540) on the plasma membrane of human glioblastoma(U-87MG) cells has been investigated. Plasma membrane was labeled with lipid specific probe 1,(4-trimethylammonium),6-diphenyl-1,3,5-hexatriene. Steady-state anisotropy, decay time and time-dependent anisotropy of TMA-DPH in U-87MG cells have been measured as a function of light dose. A decrease in the steady-state anisotropy and decay time of TMA-DPH in MC540-treated cells was observed upon light irradiation. The time-dependent anisotropy measurements showed a decrease in the limiting anisotropy (r infinity) and an increase in the rotational relaxation time (phi) of the probe upon photosensitization of cells. Analysis of these data using wobbling in cone model for probe rotation in the membrane indicated an increase in the cone angle (theta c) and a decrease in the order parameter (S). Protein specific probe N-(1-pyrene)-maleimide was used to study the effect of photosensitization on the plasma membrane proteins. An increase in the rotational relaxation time and a decrease in the ratio of excimer to monomer fluorescence intensity of PM was observed on photosensitization. Photodynamic action of MC540 also caused an inhibition of protein SH groups and Na(+)-K(+)-ATPase activity of plasma membrane. Our results demonstrate that the photodynamic action of MC540 decreases the order of the lipid bilayer and reduces the mobility of the proteins in the plasma membrane of cells.

Lipid peroxidation in photodynamically stressed mammalian cells: use of cholesterol hydroperoxides as mechanistic reporters

Photodynamic action of merocyanine 540, an antileukemic sensitizing dye, on murine L1210 cells results in the formation of lipid hydroperoxides and loss of cell viability. High-performance liquid chromatography with mercury cathode electrochemical detection was used for determining lipid oxidation products, including the following cholesterol-derived hydroperoxides: 5 alpha-OOH, 6 alpha-OOH, 6 beta-OOH, and unresolved 7 alpha, 7 beta-OOH. Among these species, 5 alpha-, 6 alpha-, and 6 beta-OOH (singlet oxygen adducts) were predominant in the early stages of photooxidation, whereas 7 alpha- and 7 beta-OOH (products of free radical reactions) became so after prolonged irradiation or during dark incubation after exposure to a light dose. These mechanistic changes were studied in a unique way by monitoring shifts in the peroxide ratio, i.e., 7-OOH/5 alpha-OOH, or 7-OOH/6-OOH. When cells (10(7)/ml) were exposed to a visible light fluence of 0.6 J/cm2 in the presence of 10 microM merocyanine 540, 7-OOH/5 alpha-OOH increased by approximately 100% after 2 h of dark incubation at 37 degrees C. The increase was much larger (approximately 250%) when cells were photooxidized after treatment with 1 microM ferric-8-hydroxyquinoline, a lipophilic iron donor, whereas no increase was observed when cells were pretreated with 100 microM desferrioxamine, an avid iron chelator/redox inhibitor. Correspondingly, postirradiation formation of thiobarbituric acid-reactive material was markedly enhanced by ferric-8-hydroxyquinoline and suppressed by desferrioxamine, as was the extent of cell killing. When added to cells after a light dose, chain-breaking antioxidants such as butylated hydroxytoluene and alpha-tocopherol strongly protected against cell killing and slowed the increase in 7-OOH/5 alpha-OOH ratio. It is apparent from these results that (1) the 7-OOH/5 alpha-OOH or 7-OOH/6-OOH ratio can be used as a highly sensitive index of singlet oxygen vs. free radical dominance in photodynamically stressed cells; and (2) that postirradiation chain peroxidation plays an important role in photodynamically initiated cell killing.

Submicrosecond phospholipid dynamics using a long-lived fluorescence emission anisotropy probe

The use of the long-lived fluorescence probe coronene (mean value of tau(FL) approximately 200 ns) is described for investigating submicrosecond lipid dynamics in DPPC model bilayer systems occurring below the lipid phase transition. Time-resolved fluorescence emission anisotropy decay profiles, measures as a function of increasing temperature toward the lipid-phase transition temperature (T(C)), for coronene-labeled DPPC small unilamellar vesicles (SUVs), are best described in most cases by three rotational decay components (phi(i = 3)). We have interpreted these data using two dynamic lipid bilayer models. In the first, a compartmental model, the long correlation time (phi(N)) is assigned to immobilized coronene molecules located in "gel-like" or highly ordered lipid phases (S-->1) of the bilayer, whereas a second fast rotational time (phi(F) approximately 2-5 ns) is associated with probes residing in more "fluid-like" regions (with corresponding lower ordering, S-->0). Interests here have focused on the origins of an intermediate correlation time (50-100 ns), the associated amplitude (beta(G)) of which increases with increasing temperature. Such behavior suggests a changing rotational environment surrounding the coronene molecules, arising from fluidization of gel lipid. The observed effective correlation time (phi(EFF)) thus reflects a discrete gel-fluid lipid exchange rate (k(FG)). A refinement of the compartmental model invokes a distribution of gel-fluid exchange rates (d(S,T)) corresponding to a distribution of lipid order parameters and is based on an adapted Landau expression for describing "gated" packing fluctuations. A total of seven parameters (five thermodynamic quantities, defined by the free energy versus temperature expansion; one gating parameter (gamma) defining a cooperative "melting" requirement; one limiting diffusion rate (or frequency factor: d(infinity))) suffice to predict complete anisotropy decay curves measured for coronene at several temperatures below the phospholipid T(C). The thermodynamic quantities are associated with the particular lipid of interest (in this case DPPC) and have been determined previously from ultrasound studies, thus representing fixed constants. Hence resolved variables are r(O), temperature-dependent gate parameters (gamma), and limiting diffusion rates (d(infinity)). This alternative distribution model is attractive because it provides a general probe-independent expression for distributed lipid fluctuation-induced probe rotational rates occurring within bilayer membranes below the phospholipid phase transition on the submicrosecond time scale.

Sulfophthalocyanines for photodynamic inactivation of viruses in blood products: effect of structural modifications

Transmission of infectious disease through blood transfusions is well known. Ultraviolet irradiation, solvents, and detergents provide a means of sterilizing noncellular blood components. However, these harsh methods are not applicable to cellular blood products. Recently, attempts have been made to sterilize biological fluids using photodynamic treatment and phthalocyanine (Pc) dyes have been advanced as photosensitizers for this purpose. We have evaluated a series of water-soluble Pc, chelated with different central metal ions, substituted to different degrees with sulfonato and t-butyl groups, for their effectiveness to reduce virus infectivity in red blood cell suspensions. Vaccinia virus cytopathogenicity was determined by endpoint serial dilutions in the CV-1 cell line. Anti-viral activity increased with the central metal ion in the following: Ga(III) < Al(III) < Zn(II), and varied inversely with the degree of sulfonation. Furthermore, addition of a t-butyl group onto the trisulfonated dyes (PcS3[t-Bu]) resulted in a 5-40-fold increase in anti-viral potency, suggesting that amphiphilicity enhances the photodynamic activity of the dye. Strong anti-viral photosensitizing properties cannot be the sole selection criterion. Of equal importance is the preservation of blood component integrity. Accordingly, the photohemolytic activity of the dyes was evaluated using the rate of hemolysis as a parameter and a toxicity index was defined. Among the most active dyes, the AlPcS3(t-Bu) complex exhibited the most favorable anti-viral properties combined with a low toxicity index. Our results suggest that trisulfophthalocyanines, bearing an additional t-butyl group to enhance amphiphilicity, are particularly promising dyes for photodynamic blood sterilization.

Factors affecting the amount and the mode of merocyanine 540 binding to the membrane of human erythrocytes. A comparison with the binding to leukemia cells

In the presence of albumin Merocyanine 540 (MC540) exhibits a very limited binding to the outer surface of the membrane of normal erythrocytes, whereas pronounced binding is observed to leukemia cells. To find out whether this difference is due to differences in the composition or structural organization of the cell membrane we analyzed effects of a number of covalent and non-covalent perturbations of the red cell membrane on the binding and fluorescence characteristics of membrane-bound MC540. It is shown that exposure of the cells to cationic chlorpromazine, neuraminidase or photodynamic treatment with AlPcS4 as sensitizer caused a limited increase (30-50%) of MC540 binding, together with a red shift of the fluorescence emission maximum and an increase of the relative fluorescence quantum yield of membrane-bound MC540. Other forms of perturbation of the membrane structure, like hyperthermia (48 degrees C) and treatments that produce a decrease of phospholipid asymmetry in addition to accelerated flip-flop, did not result in increased MC540 binding, but did cause a red shift of the fluorescence emission maximum and an increase of the relative fluorescence quantum yield. These changes in fluorescence properties indicate a penetration of the dye into more hydrophobic regions in the membrane. MC540, bound to Brown Norway myelocytic leukemia cells, exhibited a red shift of the fluorescence emission maximum and an increased relative fluorescence quantum yield as compared to MC540 bound to untreated erythrocytes. These changes were of the same order of magnitude as in photodynamically treated red blood cells. Dye binding per surface area, however, was about 3-times higher with these leukemia cells than with photodynamically treated red blood cells. This demonstrates that certain perturbations of the erythrocyte membrane evoked a MC540 binding that became qualitatively comparable to the dye binding to leukemia cells, although dye binding per surface area was still significantly lower.

Influence of dye and protein location on photosensitization of the plasma membrane

Two membrane-photosensitizing dyes were used to investigate whether selected sites in the plasma membrane vary in their sensitivity to damage by singlet oxygen (1O2*) and, if so, what factors are responsible for the variation. The relative ability of Rose bengal (RB) and merocyanine 540 (MC540), both of which localize in the plasma membrane and produce 1O2*, to photosensitize five plasma membrane functions in P388D1 cells was evaluated. The five membrane functions assessed were: plasma membrane potential, proline transport, facilitated glucose diffusion, 5'-nucleotidase activity, and dye exclusion. Photosensitization efficiency by RB varied by a factor of 188 for these membrane functions, whereas for MC540 a range of only 24 was found. RB was a more efficient photosensitizer than MC540 but the relative efficiencies varied with the membrane function. The wide range of P50 values for RB suggests that it binds selectively to membrane sites where it causes damage with high efficiency; possibly a non-1O2* mechanism is involved. In contrast, MC540 photosensitized the three membrane functions involving integral membrane proteins about equally suggesting that differences are due to small variations in the distribution of MC540 in the plasma membrane and/or variations in the inherent reactivity of the membrane targets with 1O2*. The results indicate that the lability of membrane sites to photosensitization depends both on their inherent reactivity with 1O2* and the relative location of specific protein and dye molecules.

Merocyanine dyes: effect of structural modifications on photophysical properties and biological activity

Merocyanine derivatives were prepared by structural alterations at the barbituric acid or chalcogenazole moieties. The photophysical properties of the dyes were markedly influenced by the presence of selenium rather than sulfur as a substituent at position 2 of the barbiturate. In methanol, quantum yields of both triplet state (phi T) and singlet oxygen sensitization (phi delta) were increased by over an order of magnitude, with a concomitant decrease in fluorescence, when selenium was present in the molecule. Photoisomerization, one of the dominant deactivation pathways in the sulfur- or oxygen-containing analogues, was completely absent in the selenium-containing derivatives. Efficient triplet state formation was observed for selenium-containing derivatives incorporated into L1210 cells by diffuse reflectance laser flash photolysis. Cytotoxicity studies, carried out using clonogenic assays on L1210 leukemia cells, showed a good correlation with phi T and phi delta, measured in solution. Experimental evidence provided by this paper supports a triplet state-, and probably singlet oxygen-, mediated phototoxic mechanism. Photoisomerization or singlet state mechanisms can be discounted.

Characterization of lipid hydroperoxides generated by photodynamic treatment of leukemia cells

A new technique, high-performance liquid chromatography with reductive mode electrochemical detection on a mercury drop (HPLC-EC), has been used for analyzing lipid hydroperoxide (LOOH) formation in photooxidatively stressed L1210 leukemia cells. Highly specific and sensitive for peroxides (detection limits < 0.5 pmol for cholesterol hydroperoxides and < 50 pmol for phospholipid hydroperoxides), this approach allows different classes of LOOH to be separated and determined in minimally damaged cells. L1210 cells in serum-containing growth medium were irradiated in the presence of merocyanine 540 (MC540), a lipophilic photosensitizing dye. Lipid extracts from cells exposed to a light fluence of 0.11 J/cm2 (which reduced clonally assessed survival by 30%) showed 12-15 well-defined peaks in HPLC-EC. None of these peaks was observed when cells were irradiated without MC540 or when dye/light-treated samples were reduced with triphenylphosphine prior to analysis. Three peaks of relatively low retention time (< 12 min) were assigned to the following species by virtue of comigration with authentic standards: 3 beta-hydroxy-5 alpha-cholest-6-ene-5-hydroperoxide (5 alpha-OOH), 3 beta-hydroxycholest-4-ene-6 beta-hydroperoxide (6 beta-OOH), and 3 beta-hydroxycholest-5-ene-7 alpha/7 beta-hydroperoxide (7 alpha/7 beta-OOH). Formation of 5 alpha-OOH and 6 beta-OOH (single oxygen adducts) was confirmed by subjecting [14C]cholesterol-labeled cells to relatively high levels of photooxidation and analyzing extracted lipids by HPLC with radiochemical detection. Material represented in a major peak at 18-22 min on HPLC-EC was isolated in relatively large amounts by semipreparative HPLC and shown to contain phospholipid hydroperoxides (predominantly phosphatidylcholine species, PCOOH) according to the following criteria: (i) decay of 18-22 min peak during Ca2+/phospholipase A2 treatment, with reciprocal appearance of fatty acid hydroperoxides; (ii) reduction of peroxide during treatment with reduced glutathione and phospholipid hydroperoxide glutathione peroxidase, but not glutathione peroxidase; and (iii) comigration with PCOOH standards in thin-layer chromatography. HPLC-EC analysis revealed quantifiable amounts of PCOOH and ChOOH at a light fluence that clonally inactivated < 10% of the cells, which allows for the possibility that photoperoxidative damage plays a causal role in cell killing.

Dynamics of merocyanine 540 in model biomembranes: photoisomerization studies in small unilamellar vesicles

The fluorescence lifetime, tau f, of merocyanine 540 (MC540) in small unilamellar vesicles was measured as a function of temperature and cholesterol content by using phase modulation fluorometry. These vesicles were formed by probe sonication of aqueous suspensions of egg phosphatidylcholine (PC) and cholesterol. The fluorescence lifetime of MC540 in these vesicles decreased with increasing temperature, but was independent of cholesterol. The decrease in tau f with temperature is attributed to trans-cis photoisomerization. At low temperatures, the inverse of tau f, or the fluorescence rate constant, kf, approaches a constant value of 0.45 +/- 0.02 ns-1, which corresponds to the value of the radiative rate constant, kr, of the dye. The photoisomerization rate constant, kiso, was obtained by subtracting kr from kf. The temperature dependence of kiso is well described by an Arrhenius equation, with an activation energy of 31.5 +/- 0.9 kJ mol-1. This Arrhenius behavior is rationalized in terms of the Smoluchowski limit for the Kramers theory for activated barrier crossing. The electronic spectra and kiso for MC540 in these vesicles are consistent with the dye being located in the polar headgroup region of the lipid bilayer.